1. Field of the Invention
The present invention relates to an image reading apparatus in which a hue balance of a light source is determined.
2. Description of the Related Art
General image reading apparatus include an original illuminating light source (hereinafter referred to as a light source) configured to illuminate an original so as to read an image on the original. The light source extends along a main scanning direction, and illuminates a reading target portion (line) of the original in a linear manner. Conventionally, a rare gas phosphor lamp typified by a white xenon lamp has been used as the light source. Nowadays, an LED array having a plurality of light emitting diodes (LEDs) arrayed in line at regular intervals is used as the light source in order to achieve higher power efficiency and downsizing of the light source. The LED array is disposed so as to keep a distance from the surface of the reading target original within a predetermined range.
In recent years, there has been an increasing need for higher color reproducibility of the image reading apparatus. Matters regarding image quality are mostly attributed to the light source. Further, abnormality or deterioration of the light source may largely influence the original reading performance. Therefore, many technologies have been developed to solve the problem of the abnormality and deterioration of the light source. Japanese Patent Application Laid-Open No. 2001-148003 discloses a technology of determining whether or not there is any abnormality in lighting-up characteristics of an external electrode xenon lamp. In Japanese Patent Application Laid-Open No. 2001-148003, a light intensity signal output from a CCD line sensor during a period from the start of lighting-up to the reading of a predetermined line is held in a line memory as image data of a plurality of lines in the main scanning direction, and the held image data and reference data are compared with each other. Based on the comparison result, it is determined whether or not there is any abnormality due to removal of part of a tape electrode of the xenon lamp.
Meanwhile, when an accumulated lighting-up time of the xenon lamp exceeds 500 hours, the xenon lamp has a stable light intensity level and RGB output balance. However, in the early stage immediately after the xenon lamp is replaced, the light intensity level output from the CCD line sensor may largely decline and the RGB output balance may largely change. In view of this, Japanese Patent Application Laid-Open No. 2001-144988 discloses a technology of preventing deterioration in color reproducibility due to the change in characteristics of the xenon lamp resulting from the replacement of the xenon lamp. In Japanese Patent Application Laid-Open No. 2001-144988, initial light intensity levels of RGB line sensors of the xenon lamp are detected, and based on the change in light intensity levels, it is determined whether or not the lamp has been replaced. When the lamp has been replaced, a color correction coefficient is initialized. After the initialization, the color correction coefficient is selected from a read only memory (RAM), to prevent a hue imbalance in the early stage after the replacement.
Incidentally, there is a kind of LED that is combined with a phosphor to emit white light. An LED generates a large amount of heat, and, though there is a difference depending on whether or not a heat dissipation measure is taken, deterioration of the phosphor is accordingly likely to occur. The deterioration of the phosphor may result in a color imbalance among red (R), green (G), and blue (B) of the white LED. When the RGB output balance is lost to a large extent, even if the image data is corrected in image processing, the quality of the reproduced image may decline.
In the light source, chromaticity changes because of deterioration over time. The use of the light source deteriorated over time may lead to a problem of a hue imbalance of the read image. In the technology of detecting abnormality of the light source by comparing the light intensity of the light source with the reference data, and in the technology of determining whether or not the lamp has been replaced based on the change in light intensity levels of the RGB line sensors, there is a problem that the hue balance of the light source cannot be determined.